Bathymetric distribution of beni-zuwai crab Chionoecetes japonicus in the northern part of the Sea of Japan

Surveys of beni-zuwai crab were conducted to clarify the bathymetric distribution and fecundity of beni-zuwai crab populations without the effects of bias due to fisheries. The results were compared with previous ones obtained at the heavily exploited Yamato Bank. Two sites in the northern part of the Sea of Japan, off southwest Hokkaido (200–2500 m deep) and West-Tsugaru Basin (300–1900 m deep), both of which are experiencing low exploitation levels, were surveyed in August to September, 2007. There was an abundance of morphologically immature crabs of both sexes in the depth range 1800–2100 m, and a few occurred at depths >2200 m. Immature crabs of <40 mm carapace width were found to migrate downslope to depths of 1800–2100 m and then to migrate upslope during molting for males and after maturity molting for females. Although data were lacking for depths >2000 m at the West-Tsugaru Basin, these relationships were significant for the pooled data of both localities, and they were similar to the results at the Yamato Bank. Clutch fullness of ovigerous females was negatively correlated with depth, which indicates that the low clutch fullness was not induced by fisheries, although the similar phenomenon observed at the Yamato Bank had previously been considered to be due to fishery effects.